Capacitive sensor measurement method for discrete time sampled system for in-circuit test

ABSTRACT

Disclosed is a novel method and apparatus for acquiring multiple capacitively sensed measurements from a circuit under test. Multiple digital sources are respectively connected to stimulate multiple respective first ends of multiple respective nets of interest. Respective second ends of the multiple respective nets of interest are capacitively sensed. The respective capacitively coupled signals are digitally sampled and shift correlated with respective expected digital signatures. If a high level of correlation is found for a given net, the net is electrically intact; otherwise, the net is characterized by either an open or some other fault that prevents it from meeting specification.

BACKGROUND OF THE INVENTION

Obtaining electrical measurements from nodes of an electrical device viaa capacitive sensing probe is often performed during the testing of aprinted circuit board. As known in the art, capacitive sensing is atechnique often used in determining the electrical connectivity of pins,nodes, traces, and other electrical conductors. See, for example, U.S.Pat. No. 5,696,431, Identification of Pin-Open Faults By CapacitiveCoupling to Keirn et al., U.S. Pat. No. 5,498,964, Capacitive ElectrodeSystem for Detecting Open Solder Joints in Printed Circuit Assemblies,to Kerschner et al., U.S. Pat. No. 5,254,953 to Crook et al., U.S. Pat.No. 5,241,336 to Crook et al., and U.S. Pat. No. 5,357,209 to Crook etal., all of which are incorporated by reference herein for all that theyteach.

For purposes of clarity, the description herein will use the followingterminology. As termed herein:

-   -   A “net” is an electrically conductive path between two        endpoints.    -   A “test access point” or “test point” is a location on a printed        circuit board where an external device (e.g., a probe)        electrically physically contacts a point on a net.    -   A printed circuit board (PCB) “pad” is a location on a PCB where        an integrated circuit lead or electrical component lead is        electrically connected to the PCB. (The connection between the        lead and pad is typically made via an integrated        circuit/component pin that is connected to the pin within the        integrated circuit/component package and solder between the pin        and pad).

FIG. 1 shows a test configuration illustrating a prior art capacitivesensing technique for determining the integrity of the electricallyconnectivity of an integrated circuit pin to a net. As illustrated, thetest configuration utilizes a capacitive sensing probe 2. The capacitivesensing probe 2 includes a metallic electrode that is positioned overthe integrated circuit package 4 of an integrated circuit in closeproximity to a lead conductor 7 that is connected to an integratedcircuit pin 6 inside the integrated circuit package 4. An oscillator 8supplies an alternating current signal, typically eight kilohertz (8KHz) at 100 mV. The output of the oscillator 8 is connected to a net 12(which may be a wire, a trace, a solder bump, or any other knownconductive material, and/or combination thereof) at a first end 12 a.The net 12 is connected at a second end 12 b to the integrated circuitpin 6, which is connected to lead conductor 7.

To perform a test, the integrated circuit pin 6 is stimulated (throughnet 12) by the AC signal generated by the oscillator 8. If the net 12between end 12 a at the output of the oscillator 8 and end 12 b at theintegrated circuit pin 6 is electrically intact, and the net 12 iselectrically connected to the pin 6, the AC signal applied at the firstend 12 a of the net 12 will appear on the integrated circuit pin 6 andconductive lead 7, where it will be capacitively coupled through theintegrated circuit package to the capacitive sensing probe 2. Thecapacitive sensing probe returns the capacitively coupled signal to ameasurement circuit 14. The measurement circuit 14 includes a phasesynchronous voltmeter 10 that receives the AC source signal 5 from theoscillator 8 and the capacitively coupled signal 3 from the probe 3. Thephase synchronous voltmeter 10 attempts to phase synchronize thecapacitively coupled signal 3 with the AC source signal 5. The voltmeter10 measures not only the amplitude of the capacitively coupled signal 3from the probe 3 but also its phase relative to the source signal 5. Thevoltmeter rejects noise by integrating the signal over multiple cyclesof the source frequency. Phase information is used to differentiatebetween capacitive, resistive, and inductive impedances.

If the amplitude of the measured impedances falls within a predeterminedacceptable range (as calculated based on the board design or as obtainedfrom measurements from a known good board), the integrated circuit pin 6is considered to be properly electrically connected (e.g., soldered,wire-bonded, etc.).

Open circuits are a common defect in the manufacture of PCB assemblies,typically occurring as a result of poor solder bonds, incomplete traces,and/or missing devices that are either never loaded onto the board orwhich fall off during the assembly process. The capacitive sensingtechnique is often used in PCB testers for detecting open connections ona PCB assembly. A PCB tester typically includes a number of testerinterface pins that are arranged on a face of the tester in a predefinedconfiguration. Because the positions of the tester interface pins areunchangeable, a test fixture customized to the particular design of thePCB under test is typically required to interface between the testerinterface pins of the tester and test points on the PCB under test. Thetest fixture includes circuitry (e.g., wires, probes) that electricallyconnects various tester interface pins on the tester to various testaccess points on the PCB under test.

Test access points of interest may be stimulated by an AC source in thetester. In this regard, the AC source in the tester may be connected tovarious tester interface pins via configurable relays within the tester.Thus, stimulation of a given test access point may therefore be achievedby configuring the relays to connect the AC source to a given testerinterface pin that connects to the test access point of interest on thePCB via the test fixture. By design, the test access point on a PCBshould be connected to a component under test (e.g., and integratedcircuit pin) via a net. The electrically integrity of the net andnet-to-component connection may be tested by using the capacitivesensing technique previously described.

In large testers, the number of available AC reference sources may belimited. Thus, testing of a complete PCB may require many relayconfiguration cycles in order to connect each of the nets under test toan available AC source. As known in the art, the relay configuration andsettling time can take a significant amount of time relative to the timerequired to obtain the capacitively coupled measurement. Since a PCB cancontain hundreds or thousands or more nets and net-to-componentconnections to be tested, this relay configuration procedure can proveto be too costly in terms of test time when only a limited number of ACsources are available for net stimulation.

It will be recognized that although the availability of many, or evenmultiple, AC sources in a large tester is often limited or non-existent,in such testers there is often a proliferation of digital sourcesavailable due to the use of the tester in performing digital functionaltests. When configuring the tester for digital functional test, thetester often has the ability to connect all or many of its testerinterface pins to a corresponding digital source in the tester in anearly one-to-one mapping. Thus, all relay connections could beperformed in parallel and the configuration time could be reduced to therelay settling time of the slowest relay if the digital sources could beused in place of the AC source(s) of the tester. However, because thedigital sources generate no phase information, in the past digitalsources have not been used to obtain capacitively sensed measurementsbecause the digital sources cannot be phase synchronized with thecapacitively coupled signal. It would therefore be desirable to have amethod that would allow the use of ubiquitously available digitalsources in a printed circuit board tester to allow faster acquisition ofmultiple capacitively sensed measurements.

SUMMARY OF THE INVENTION

The present invention is a novel technique for allowing the acquisitionof multiple capacitively sensed measurements that utilizes testerhardware for generating digital stimulation signals, tester pins fordelivering the stimulation signals to nodes of interest on the PCB undertest, and digital signal processing (DSP) correlation and detectionfunctionality from which the presence or absence of electricalconnectivity between two nodes of interest can be derived. In accordancewith a preferred embodiment of the invention, the capacitive sensingtechnique involves the steps of: for each of a plurality of nodes ofinterest on a PCB under test, stimulating a first end of a net undertest with a known signal, capacitively sensing a signal on a componentthat should be connected to the net at a second end of the net,correlating the capacitively sensed signal with the known signal, anddetermining whether the level of correlation meets a pre-determinedlevel of correlation.

The invention advantageously allows the use of digital stimulationsources due to the fact that it does not utilize specific phaseinformation (which is unavailable for digital tester sources) tocorrelate the capacitively coupled signal with the source signal.Because the digital stimulation signal utilizes a known pattern, thesensed signal can be correlated with the stimulation signal based onpattern recognition rather than specific phase information. Anotheradvantage of the invention is that by allowing the use of ubiquitouslyavailable digital stimulation sources normally used for functional testin the tester, the setup time for the acquisition of multiplecapacitively sensed measurements is significantly decreased.

BRIEF DESCRIPTION OF THE DRAWINGS

A more complete appreciation of this invention, and many of theattendant advantages thereof, will be readily apparent as the samebecomes better understood by reference to the following detaileddescription when considered in conjunction with the accompanyingdrawings in which like reference symbols indicate the same or similarcomponents, wherein:

FIG. 1 is a schematic block diagram illustrating the prior art testconfiguration for acquiring capacitive sensor measurement;

FIG. 2 is a schematic block diagram illustrating a first embodiment of atest configuration implemented in accordance with the invention foracquiring a capacitive sensor measurement without specific phaseinformation;

FIG. 3 is a top cutaway view of an integrated circuit chip;

FIG. 4 is a top cut away view of the integrated circuit chip of FIG. 3and a capacitive sensing probe;

FIG. 5 is a side cut away view of the integrated circuit chip andcapacitive sensing probe of FIG. 5;

FIG. 6 is a top, front perspective view of an exemplary embodiment of acapacitive sensing probe;

FIG. 7 shows a side cut-away view of the capacitive sensing probe ofFIG. 6;

FIG. 8 is a flowchart illustrating a method for performing a test fordetecting open or sub-specification conditions of a net under test;

FIG. 9 is a flowchart illustrating an alternative method for performinga test for detecting open or sub-specification conditions of a net undertest;

FIG. 10 is a schematic block diagram of a second embodiment testconfiguration for acquiring multiple capacitive sensor measurementsimplemented according to the invention;

FIG. 11 is a flowchart illustrating another alternative method forperforming a test for detecting open or sub-specification conditions ofa net under test;

FIG. 12 is a schematic block diagram of a third embodiment testconfiguration for acquiring multiple capacitive sensor measurementsimplemented according to the invention;

FIG. 13 is a top cutaway view of the integrated circuit chip andcapacitive sensing probes of FIG. 12;

FIG. 14 is a flowchart illustrating another alternative method forperforming a test for detecting open or sub-specification conditions ofa net under test;

FIG. 15 is a schematic block diagram of a fourth embodiment testconfiguration for acquiring multiple capacitive sensor measurementsimplemented according to the invention;

FIG. 16 is a top cutaway view of the integrated circuit chip andcapacitive sensing probes of FIG. 15;

FIG. 17 is a schematic block diagram of a portion of an in-circuit testsystem employing the capacitive sensing techniques of the invention;

FIG. 18 is a waveform diagram illustrating example waveforms for thesystem of FIG. 17; and

FIG. 19 is a flowchart illustrating an exemplary method for correlatinga digital representation of a capacitively sensed signal with anexpected digital signature without the use of phase information.

DETAILED DESCRIPTION

A novel technique for acquiring multiple capacitively sensedmeasurements from a circuit under test is described in detailhereinafter. Although the invention is described in terms of specificillustrative embodiments, it is to be understood that the embodimentsdescribed herein are by way of example only and that the scope of theinvention is not intended to be limited thereby.

Turning now to the invention, FIG. 2 is a schematic block diagram of afirst embodiment of a test configuration for acquiring capacitivelysensed measurements implemented according to the invention. In thisconfiguration, a capacitive sensing probe 20 comprising a metallicelectrode 21 is placed on top of the package 34 of an integrated circuitchip 30. A net 42 a (which may be a wire, a trace, a solder bump, or anyother known conductive material, and/or combination thereof) ischaracterized by a first end 41 a connected to the output of anoscillating digital source 40 and a second end 43 a connected to a pin36 a of the integrated circuit chip 30.

Digital square-wave source 40 generates a reference signal (or“stimulus”) 27. Analog-to-digital (A/D) sampling circuit 50 is connectedto receive a capacitively coupled signal 23 sensed by probe 20. Thesampling circuit 50 samples at least two times (2×) the frequency of thereference signal 27. In the preferred embodiment, the sampling rate isten times (10×) the frequency of the reference signal 27 to allow a goodrepresentation 25 of the signal 23. Once a digital representation 25 ofthe signal 23 is captured, a correlation function 60 performs acorrelation algorithm to correlate the sampled signal 25 with anexpected digital signature 29 (i.e., the digital signal of the referencesignal 27). Since the digital signature (i.e., pattern) of the referencesignal 27 is known, no phase information is required. The correlationresults are indicative of the integrity of the net and net-to-pinconnection.

FIG. 3 shows a top cutaway view of the integrated circuit chip 30. Asillustrated, the integrated circuit chip 30 comprises an integratedcircuit 31 packaged in an integrated circuit package 34. The integratedcircuit 31 includes signal pads that must be made available outside ofthe integrated circuit package 34. Therefore, each pin 36 a, 36 b, 36 c,36 d, 36 e, 36 f, 36 g, 36 h is connected to an internal lead conductor35 a, 35 b, 35 c, 35 d, 35 e, 35 f, 35 g, 35 h which connects the pin 36a, 36 b, 36 c, 36 d, 36 e, 36 f, 36 g, 36 h to a location just adjacentthe integrated circuit 31. There, a small wire spans between eachrespective conductor 35 a, 35 b, 35 c, 35 d, 35 e, 35 f, 35 g, 35 h andits respective integrated circuit pad on the integrated circuit 31.

FIG. 4 is a top cut away view, and FIG. 5 is a side cut away view, ofthe integrated circuit chip 30 and capacitive sensing probe 20(indicated by dashed lines). One plate of the capacitor formed by theprobe 20 is the probe electrode 21 (illustrated by dashed lines), hereshown with probe electrode 21 positioned over the top of a respectiveinternal lead conductor 35 a. The other plate of the capacitor formed bythe probe 20 is the internal lead conductor 35 a. Although the capacitorcreated between the probe electrode 21 and internal lead conductor 35 ain this manner is small, it is sufficient to conduct a signal from theinternal lead conductor 35 a to the probe electrode 21.

The capacitive sensing probe 20 may be used in conjunction with a testrobot (not shown) to test the connection of each lead conductor 35 a, 35b, 35 c, 35 d, 35 e, 35 f, 35 g, 35 h of the integrated circuit chip 30individually. For this implementation, the capacitive sensing probe 20may include a support mechanism 24 (FIG. 5) to enable a mechanical arm26 of the robot to grab the test probe. The robot is then programmed forthe coordinates of each lead conductor 35 a, 35 b, 35 c, 35 d, 35 e, 35f, 35 g, 35 h.

FIG. 6 is a top, front perspective view of an example capacitive sensingprobe 200 and FIG. 7 shows a side cut-away view of the capacitivesensing probe 200 that may be used to implement the capacitive sensingprobe 20. As shown in FIGS. 6 and 7, the capacitive sensing probe 200includes a capacitive plate 202, a guard plate 204, an active buffercircuit 208, a guard electrode spring pin 201 and a signal electrodespring pin 212. The capacitive plate 202 and the guard plate 204 in theillustrative embodiment are preferably made of copper, but can be madeof any electrically conductive material. The capacitive plate 202 andthe guard plate 204 are separated by a dielectric 205, such as glassfilled plastic or any other insulative material. The dielectric isapproximately 0.04 inches thick. It should be understood that if thedielectric 205 is too thin, the capacitive reading will be distortedupward, and if the dielectric is too thick, the shielding effect of theguard plate will be reduced and stray system capacitance will bedetected. The capacitive plate 202 in the present invention forms acapacitor with a conductive lead 35 a–35 h internal to the integratedcircuit (see FIGS. 4 and 5). The capacitive plate 202 of the test probe206 is electrically coupled to an active buffer circuit 208, which islocated on the top surface of the dielectric and surrounded by the guardplate 204. The capacitive plate 202 is connected to the buffer circuit208 at a location 203 (see FIG. 7). The amplification of the signal bythe buffer circuit 208 which is in close proximity to the capacitiveplate 202 where the signal is received helps to significantly optimizethe signal to noise ratio, thereby decreasing the effect of system noiseand stray capacitance. However, this can be accomplished by other meanssuch as shielding by means of a coaxial cable or a conductive foamsurrounding the spring pins 210 and 212.

During manufacturing, the dielectric 205 is deposited on the capacitiveplate 202 and then the guard plate 204 is deposited on the dielectric.Next, the guard plate is etched down to the dielectric 205 to formtraces for a buffer circuit 208 and a groove 226 is etched all the wayaround the buffer circuit area to electrically isolate the buffercircuit from the guard plate. During manufacturing, the buffer circuit208 is mounted to the traces formed from the guard plate by using a chipon board procedure. The buffer circuit is electrically connected by apin in socket connector 228 to a standard signal electrode spring pin212, which acts as an electrical coupling means to a measuring device.The guard plate 204 is electrically connected via connector 230 to aguard electrode spring pin 210, which electrically couples the guardplate to system ground or a controlled voltage source.

Spring pins 210 and 212 give the test probe z-axis travel, which allowsfor intimate coupling with the integrated circuit component to betested, regardless of the height of the component. Also, when theinvention is used to test an entire circuit board, as in FIG. 17, thez-axis travel of the spring pins permit all of the test probes 206 tointimately contact the corresponding components, even if the heights ofthe components are not uniform. This z-axis travel can be accomplishedby other means such as hydraulic pins with z-axis travel. Moreover, thez-axis travel is not necessary, as long as the test probe is apredetermined distance from the integrated circuit package so that thereis a predetermined range of results.

The spring pins 210 and 212 are attached to standard connectors 228 and230 via pin in socket coupling. The connectors 228 and 230 are solderedto the buffer circuit 208 and to the guard plate 204, respectively. Thepin in socket coupling between the spring pins and the connectors isflexible enough to create a slight x,y plane swivel, which allows thetest probe 206 to conform to the top surface of the integrated circuitto be tested, if the bottom surface of the test probe 206 is angularlyoffset from the top surface of the integrated circuit component, thusallowing a substantially uniform distance to be maintained between thetest probe 206 and the integrated circuit package 34.

FIG. 8 describes the steps involved in performing a test for detectingopen or sub-specification conditions along a net 42 a and/or between thenet 42 a and pin 36 a. As illustrated therein, to perform a test, thedigital source 40 is connected in a step 101 to a first end 41 a of anet 42 a of interest. If the net 42 a is electrically intact andconductively connected to pin 36 a at the second end 43 a of the net 42a, full conductive continuity along the net 42 a should exist and shouldgenerate a predetermined capacitively coupled signal level when thecapacitive coupling probe 20 is placed within a predetermined proximityto the respective internal lead conductor 35 a of the integrated circuitpackage 34.

In a step 102, the first end 41 a of the net 42 a is stimulated with anoscillating digital reference signal 27 by the digital source 40.Capacitive sensing probe 20 then senses a signal from the second end 43a of the net 42 a. In the illustrative embodiment, integrated circuitpin 36 a should be connected to the second end 43 a of the net 42 a. Ifthe net 42 a between the digital source 40 at end 41 a and theintegrated circuit pin 36 a at second end 43 a is electrically intactand conductively connected to the integrated circuit pin 36 a, thedigital signal present on the integrated circuit pin 36 a will bepresent on the internal lead conductor 35 a, which will be capacitivelycoupled through the integrated circuit package 34 to the capacitivesensing probe 20. Thus, the probe electrode 21 operates as onecapacitive plate of the probe 20 and the internal lead conductor 35 aoperates as the other capacitive plate for the probe 20.

In step 104, the capacitively coupled signal 23 is digitally sampled toconvert it from an analog signal to a digital signal 25. In theillustrative embodiment, an analog-to-digital (A/D) sampling circuit 50receives the capacitively coupled signal 23 from the capacitive sensingprobe 20 and converts it to a digital signal 25, preferably at asampling rate of ten times (10×) the frequency of the digital sourcesignal.

In step 105, the digital representation 25 of the capacitively coupledsignal 23 is correlated with an expected digital signature 29 of thereference signal 27 (without specific phase information of the digitalsource signal 27). In the illustrative embodiment, a signal correlator60 performs shift correlation on the digital representation 25 of thecapacitively coupled signal 23 with the expected digital signature 29 ofthe reference signal 27. If the continuity of the net under test 42 a isintact and conductively connected to pin 36 a, the digitalrepresentation 25 of the capacitively coupled signal 23 will have astrong correlation with the expected digital signature 29 at some shiftof the correlation. If the continuity of the net under test 42 a is pooror open, the digital representation 25 of the capacitively coupledsignal 23 will have little or no correlation with the expected digitalsignature 29 at any shift of the correlation.

FIG. 9 describes one method 120 for performing a test for sequentiallydetecting open or sub-specification conditions of multiple nets 42 a–42d of a circuit under test according to the embodiment of FIG. 2. Asillustrated therein, to perform a test, the digital source 40 isconnected in a step 121 to the first end 41 a of a first net 42 a to betested. Because in this embodiment there is only one capacitive sensingprobe 20 available to take the capacitively coupled signal measurements,the method 120 sequentially obtains the capacitively coupled signalsfrom the respective internal lead conductors 35 a–35 d of the integratedcircuit package 34. Accordingly, the method 120 makes a determination instep 121 as to whether there are any unprocessed nets 42 a, 42 b, 42 c,42 d remaining to be processed. If so, in step 122 the next unprocessednet 42 a, 42 b, 42 c, 42 d is selected. In a step 123, the digitalsource 40 is connected to the first end 41 a–41 d of the selected net 42a, 42 b, 42 c, 42 d. In a step 124, the first end 41 a, 41 b, 41 c, 41 dof the selected net 42 a, 42 b, 42 c, 42 d is stimulated by digitalsource 40. The capacitive sensing probe 20 then capacitively senses asignal from the internal lead conductor 35 a, 35 b, 35 c, 35 d thatshould be electrically connected to the selected net 42 a, 42 b, 42 c,42 d through the package 34 in step 125. If the selected net 42 a, 42 b,42 c, 42 d is electrically intact and electrically connected to itsrespective pin 36 a, 36 b, 36 c, 36 d, the digital oscillating signalpresent on the integrated circuit pin 36 a, 36 b, 36 c, 36 d will becapacitively coupled through the integrated circuit package 34 to thecapacitive sensing probe 20.

In step 126, the sensed signal 23 is converted from an analog signal toa digital signal 25. In the illustrative embodiment, ananalog-to-digital (A/D) sampling circuit 50 receives the capacitivelycoupled signal 23 sensed by the capacitive sensing probe 20 and convertsit to a digital representation 25. Steps 122 through 126 are thenrepeated until measurements for each of the nets 42 a, 42 b, 42 c, 42 dof interest have been taken. The digital representation 25 of eachcapacitively coupled signal 23 is correlated by signal correlator 60with its respective expected digital signature. This step may occurafter each iteration of the loop (step 127 a), or alternatively thedigital representations 25 may be stored after each loop and thencorrelated after the digital representations of all nets of interesthave been obtained (step 127 b). If the continuity of a given net 42 a,42 b, 42 c, 42 d is intact and conductively connected to its respectivepin 36 a, 36 b, 36 c, 36 d, its corresponding extracted signal 25 willhave a strong correlation with its expected digital signature at someshift of the correlation. If the continuity of a net 42 a, 42 b, 42 c,42 d is poor or open, its corresponding extracted signal 25 will havelittle or no correlation with its expected digital signature at anyshift of the correlation.

FIG. 10 is a schematic block diagram of a second embodiment testconfiguration for acquiring multiple capacitive sensor measurementsimplemented according to the invention. In this configuration, multiplenets 45 a, 45 b, 45 c, 45 d of interest are tested. In the exampleshown, each net 45 a, 45 b, 45 c, 45 d (which may be a wire, a trace, asolder bump, or any other known conductive material, and/or combinationthereof) is characterized by a first end 41 a, 41 a, 41 b, 41 c, 41 dconnected to the output of a respective oscillating digital source 40 a,40 b, 40 c, 40 d, and a second end 43 a, 43 b, 43 c, 43 d connected to arespective integrated circuit pin 36 a, 36 b, 36 c, 36 d.

In the embodiment of FIG. 10, the area of the metallic electrode plate21 of the capacitive sensing probe 20 is sized to cover only a portionof the conductor 35 a, 35 b, 35 c, 35 d, 35 e, 35 f, 35 g, 35 hconnected to a single pin 36 a, 36 b, 36 c, 36 d, 36 e, 36 f, 36 g, 36 hof the integrated circuit package 34.

FIG. 11 shows a method 130 for performing a test for detecting open orsub-specification conditions of multiple nets 42 a–42 h of a circuitunder test utilizing the configuration of FIG. 10. As illustrated inFIG. 11, to perform a test, respective digital sources 40 a–40 h areconnected in a step 131 to respective first ends 41–41 h of respectivenets 42 a–42 h under test. Because in this embodiment there is only onecapacitive sensing probe 20 available to take measurements on each ofthe respective nets 42 a–42 h, the method 130 obtains the signal samplessequentially. Accordingly, a determination is made in step 132 as towhether there are any unprocessed nets 42 a–42 h remaining to beprocessed. If there are any unprocessed nets 42 a–42 h, in step 133 thenext unprocessed net 42 a–42 h is selected. In a step 134, the first end41 a–41 h of the selected net 42 a–42 h is stimulated by its respectiveoscillating source 40 a–40 h. In step 135, the capacitive sensing probe20 then capacitively senses a signal present on the respective internallead conductor 35 a–35 h that should by design be connected to the firstend 41 a–41 h of the selected net 42 a–42 h through the package 34. Ifthe respective net 42 a–42 h is electrically intact and electricallyconnected to its respective pin 36 a–36 h, the digital signal present onthat respective pin 36 a–36 h will be capacitively coupled through theintegrated circuit package 34 to the capacitive sensing probe 20. Instep 136, the sensed signal 23 is converted from an analog signal to adigital signal 25. In the illustrative embodiment, an analog-to-digital(A/D) sampling circuit 50 receives the respective capacitively coupledsignal 23 sensed by the capacitive sensing probe 20 and converts it to adigital representation 25. Steps 132 through 136 are then repeated forall nets 42 a–42 h of interest. The digital representation 25 of eachcapacitively coupled signal 23 is correlated by signal correlator 60with its respective expected digital signature. This step may occurafter each iteration of the loop (step 137 a), or alternatively thedigital representations 25 may be stored after each loop and thencorrelated after the digital representations of all nets of interesthave been obtained (step 137 b). If the continuity of a given net 42 a,42 b, 42 c, 42 d is intact and conductively connected to its respectivepin 36 a, 36 b, 36 c, 36 d, its corresponding extracted signal 25 willhave a strong correlation with its expected digital signature at someshift of the correlation. If the continuity of a net 42 a, 42 b, 42 c,42 d is poor or open, its corresponding extracted signal 25 will havelittle or no correlation with its expected digital signature at anyshift of the correlation.

FIG. 12 is a schematic block diagram of a third embodiment testconfiguration for acquiring multiple capacitive sensor measurementsimplemented according to the invention. In this configuration, multiplenets 42 a–42 h of interest are tested. In the example shown, each net 42a–42 h (which may be a wire, a trace, a solder bump, or any other knownconductive material, and/or combination thereof) is characterized by afirst end 41 a–41 h respectively connected to the output of a respectiveoscillating digital source 40 a–40 h, and a second end 43 a–43 hrespectively connected to a respective integrated circuit pin 36 a–36 h.

FIG. 13 shows a top cutaway view of the integrated circuit chip 30 andcapacitive sensing probes 20 a–20 h of FIG. 12 (indicated by dashedlines). As illustrated, a different respective capacitive sensing probe20 a, 20 b, 20 c, 20 d, 20 e, 20 f, 20 g, 20 h is respectively alignedover a different respective internal lead conductor 35 a, 35 b, 35 c, 35d, 35 e, 35 f, 35 g, 35 h of the integrated circuit chip 30. The area ofthe metallic electrode plate of each capacitive sensing probe 20 a, 20b, 20 c, 20 d, 20 e, 20 f, 20 g, 20 h is sized to cover only a portionof its respective conductor 35 a, 35 b, 35 c, 35 d, 35 e, 35 f, 35 g, 35h so as not to overlap onto neighboring conductors 35 a, 35 b, 35 c, 35d, 35 e, 35 f, 35 g, 35 h.

In the embodiment shown in FIGS. 12 and 13, a different respectivecapacitive sensing probe 20 a–20 h capacitively senses a separate signalfrom each respective conductor 35 a–35 h. These signals can be returnedto the signal processing circuitry in parallel, or alternatively can becollected sequentially through the use of a multiplexer 86 andsequencing controller 84 (shown in dashed lines in FIG. 12).

FIG. 14 illustrates a method 110 for performing a test for detectingopen or sub-specification conditions of multiple nets 42 a–42 d of acircuit under test utilizing the configuration of FIG. 12 when thecapacitively sensed signals are collected in parallel. As illustrated inFIG. 14, to perform a test, each digital source 40 a–40 d is connectedin a step 111 to a respective first end 41 a–41 d of its respective net42 a–42 d. In a step 112, the respective first end 41 a–41 d arestimulated by respective digital sources 40 a–40 d. Respectivecapacitive sensing probes 20 a–20 d then each sense a signal from therespective second end 46 a–46 d of its respective net 42 a–42 d. In step104, the respective sensed signals are each converted from a respectiveanalog signal to a respective digital signal. In step 115, eachrespective digital representation of the respective sensed signals iscorrelated with a respective expected digital signature. If thecontinuity of the respective net under test is intact, the respectiveextracted signal will have a strong correlation with its respectiveexpected digital signature at some shift of the correlation. If thecontinuity of the respective net under test is poor or open, therespective extracted signal will have little or no correlation with itsrespective expected digital signature at any shift of the correlation.It is to be understood that depending on the implementation, the signalconversion and processing can be performed in parallel or in serial.

FIG. 15 is a schematic block diagram of a fourth embodiment testconfiguration for acquiring multiple capacitive sensor measurementsimplemented according to the invention, and FIG. 16 is a top cutawayview of the integrated circuit chip 30 and a segmented capacitivesensing probe 120, indicated by dashed lines. In this embodiment, thesegmented capacitive sensing probe 120 comprises a plurality ofindividual probe plates 120 a, 120 b, 120 c, 120 d, 120 e, 120 f, 120 g,120 h, indicated in FIG. 16 by dashed lines, each of which is alignedover a single internal lead conductor 35 a, 35 b, 35 c, 35 d, 35 e, 35f, 35 g, 35 h in the integrated circuit chip 30 when the segmentedcapacitive sensing probe 120 is aligned over the integrated circuitpackage 34. The area of each individual probe plate 120 a, 120 b, 120 c,120 d, 120 e, 120 f, 120 g, 120 h is sized to cover only a portion ofits respective conductor 35 a, 35 b, 35 c, 35 d, 35 e, 35 f, 35 g, 35 hso as not to overlap onto neighboring conductors 35 a, 35 b, 35 c, 35 d,35 e, 35 f, 35 g, 35 h and to allow each individual probe plate 120 a,120 b, 120 c, 120 d, 120 e, 120 f, 120 g, 120 h to separately sense asignal from its respective conductor 35 a, 35 b, 35 c, 35 d, 35 e, 35 f,35 g, 35 h. The leads of the individual probe plates 120 a, 120 b, 120c, 120 d, 120 e, 120 f, 120 g, 120 h may be connected to the signalprocessing circuitry 50, 60 in parallel to allow the signals to beprocessed in parallel. Alternatively, the leads of the individual probeplates 120 a, 120 b, 120 c, 120 d, 120 e, 120 f, 120 g, 120 h may beconnected to a multiplexer 84 that is controlled by a sequencingcontroller 86, as shown in dashed lines, and the signals are collectedone at a time (i.e., serially).

When capacitively sensed measurements in the embodiment of FIGS. 15 and16 are collected in parallel, the method described in FIG. 14 may befollowed to perform a test for detecting open conditions orsub-specification electrical connections on the circuit under test. Whencapacitively sensed measurements in the embodiment of FIGS. 15 and 16are collected serially, the method described in FIG. 11 may be followedto perform the test.

The results of correlation may be used in various ways. For example, abasic go/no-go test may be performed to determine whether an opencircuit exists on a net. An open exists if the correlator 60 finds nocorrelation with the expected digital signature. However, even if somecorrelation exists, indicating that the net does not have a completeopen, the electrical integrity of the net may not be sufficient to meetthe specifications of the circuit. For example, if the net comprises aprinted circuit board trace, and the trace, for whatever reason, is verythin somewhere along the path, while technically net would not becharacterized by an open, the amount of current the trace can conductmay be limited due to the “thin area” of the trace such that the currentdrive of the trace does not meet the board specifications. Accordingly,the strength of the correlation detected by the correlator 60 may becompared to a minimum correlation strength threshold to determinewhether the net meets minimum conductive integrity requirements.Accordingly, the results of the correlator 60 may be utilized in variousways to meet the needs of the specific application.

Turning now to consider the use of the invention in mass productionprinted circuit board (PCB) assembly and testing, there is shown in FIG.17 a portion of an in-circuit test system 300 employing, forillustrative non-limiting purposes only, several capacitive couplingprobes 320 a, 320 b, 320 c implemented in accordance with the invention.As illustrated, the in-circuit test system 300 includes a tester 330 anda fixture 340 for seating a PCB under test 302. Due to the close spacingof the tester interface pins, nodes of the PCB under test, and smallsize of the components under test, only a small edge portion of thetester/fixture/PCB under test combination is shown for ease ofillustration.

Tester 330 includes a plurality of tester interface pins 331 arranged inan array along the top side of the tester 330. Tester 330 includestester hardware 335 to make or to not make electrical connectionsbetween digital sources 361 within the tester and the various testerinterface pins 331. To this end, each tester interface pin 331 isconnectable to or isolated from a digital source 361 by one or morecorresponding relays 334.

Mounted on top of the tester 330 and over the tester interface pins 331is the test fixture 340. The test fixture 340 interfaces the testerinterface pins 331 to test access points 303 a–303 e on the PCB undertest 302.

The fixture 340 is configured with a number of capacitive couplingprobes 320 a, 320 b, 320 c. For convenience of illustration and clarityof the invention, only three such capacitive coupling probes 320 a, 320b, 320 c are shown; however, it will be appreciated by those skilled inthe art that a conventional in-circuit tester may have many more suchprobes. In the illustrative embodiment, the probes 320 a, 320 b, 320 care mounted to the fixture top 342 such that the capacitive plate 310 a,310 b, 310 c of each apparatus 320 a, 320 b, 320 c precisely aligns overits corresponding component under test 306 a, 306 b, 306 c when the PCB302 is properly mounted in the fixture 340.

In the preferred embodiment, the fixture 340 may include one capacitivecoupling probe 320 for each integrated circuit, capacitor, resistor, orother component of interest on the printed circuit board 302.Accordingly, a large number of capacitive coupling probes 320 may berequired. For this reason, it may be desirable to multiplex the controlsignals and analog response signals 342 going between the tester andthe-capacitive coupling probes 320 a–320 c to reduce the number of wiresbetween the tester 330 and fixture 340.

The capacitive coupling probes 320 a, 320 b, 320 c are used to performin-circuit capacitive measurement tests. An example of a capacitivesensing probes ideal for use in the present invention is AgilentTechnology's TestJet™ probe, which is described in detail in U.S. Pat.No. 5,254,953 to Crook et al., U.S. Pat. No. 5,241,336 to Crook et al.,U.S. Pat. No. 5,498,964 to Kerschner et al., U.S. Pat. No. 5,357,209 toCrook et al., and U.S. Pat. No. 5,696,431 to Keirn et al.

When an in-circuit capacitive measurement test is to be executed, theconfiguration hardware 335 configures the relays 334 such that digitalsources 361 are electrically connected to respective tester interfacepins 331 that electrically connect through the fixture 340 to the testaccess points 303 a–303 c on the bottom side 305 of the PCB under test302. The capacitive coupling probes 320 a–320 c are positioned overcomponents under test. The capacitive coupling probes 320 a–320 c areused to determine whether the nets between the test access pointstructures 303 a–303 c and a respective component under test on the PCB302 are electrically intact. The sequencer controller 366 enables thedigital sources 361 to begin stimulating their respective test accesspoints 303 a–303 c. The sequencer controller 366 configures themultiplexer card 346 to route capacitively sensed signals from therespective capacitive sensing probes 320 a–320 c to the tester 330according to a predetermined order. As the capacitively coupled signalsare received back from the probes 320 a–320 c, measurement circuitry 362acquires the signals and passes them through an anti-aliasing filter 363prior to analog-to-digital conversion by A/D converter 364. Each of theconverted digital measurements are preferably stored in a respectivedesignated area 365 a, 365 b, 365 n in memory 365.

A correlation function 340 operates to correlate each of the storedconverted digital measurements in memory 365 with expected digitalsignatures for those measurements.

In each of the above embodiments, the method of correlation depends onthe type of reference signal 27, or “stimulus”, generated by the digitalsource 40. The stimulus 27 may be either a square-wave stimulus or acoded pulse-train stimulus.

If the stimulus 27 is a square-wave stimulus, then correlation of thedigital representation 25 of the captured capacitively coupled signal 23with the expected digital signature of the stimulus is preferablyachieved using cross-correlation. In the cross-correlation method, thesignal used for stimulus is a simple square wave or pulse train. Thetime ordered measurements taken by the A/D converter are multiplied by atime ordered mathematical representation of the square-wave stimulus(i.e. +1 or −1). Correlation is the normalized sum of all thesemeasurements. This type of correlation function is calledcross-correlation. Cross-correlation is a well-known technique incommunications. It is used widely in spread spectrum systems where aninformation signal is spread over a wide frequency band to form a spreadspectrum signal, transmitted as a spread spectrum signal, and recoveredby remapping the received spread spectrum signal into the originalinformation bandwidth. The information to be transmitted, (i.e., thestimulus signal) may be embedded in the spread spectrum signal accordingto several methods. These methods are described in Dixon, R. C., “SpreadSpectrum Systems”, Wiley-Interscience, 1976, which is incorporatedherein by reference for all that it teaches.

In one embodiment, if the stimulus 27 is a square-wave stimulus, thecorrelation function 60 correlates the capacitively coupled signal 23with phase (i.e., time) shifted representations of the square wavestimulus signal 27. The correlation function 60 then performs a searchto determine the phase at which the magnitude of the correlationfunction peaks. This peak magnitude will be linearly related to theamount of coupling between the stimulus probe and the capacitive sensingprobe. It will be compared to a threshold value in order to determinethe presence or absence of an electrical connection between thestimulating node and the lead frame of the tested IC.

FIG. 18 is a waveform diagram illustrating example capacitively coupledsignal waveforms 401, 402, 403 and corresponding expected digitalsignature waveform 404 and digital representation waveform 405 acquiredwhen the stimulus is a square-wave stimulus. For example, in theembodiment of FIG. 17, the digital sources 361 may be synchronized tooutput an identical square wave signal, shown as waveform 404. Waveforms401, 402, and 403 may for example correspond to capacitively coupledsignals collected by each of probes 320 a, 320 b, and 320 c. Waveform405 illustrates the sequential digital representation of thecapacitively sensed signals received by each of probes 320 a, 320 b, and320 c. As illustrated, the digital representation of the capacitivelysensed signals received by each of probes 320 a, and 320 c match theexpected digital signature (i.e., the waveform matches the input at someshift of correlation). The digital representation of the capacitivelysensed signal collected by probe 320 b, however, does not match theexpected digital signature. The correlation function 340 will thereforebe unable to correlate the measurement with its expected digitalsignature, indicating an open connection on the component probed byprobe 320 b.

In an alternative embodiment, the capacitively coupled signal may becorrelated with itself. This is referred to as auto-correlation. In thisapproach, since the rate at which the stimulus waveform (straightfrequency, chirp signal, or other repeats is known, the period of thestimulus signal is known. The auto-correlation algorithm will detectcorrelations as the data is shifted across itself at the frequency ofthe period of the stimulus signal.

In particular, in an example embodiment, the auto-correlation algorithmsearches for multiple correlations (e.g., 10–20 or more) spaced apart atthe “expected” stimulus frequency.

It should be noted that when using auto-correlation, some cautionarymeasures should be taken to ensure that the correlated signal is that ofthe capacitively coupled signal measurement and not spurious noise. Asknown in the art, any repetitive signal at any frequency (i.e., not justthe pulse train and not just the stimulus frequency) that is present atthe input of the A/D converter will correlate with itself. The classicexample of repetitive spurious noise is line frequency noise (60 Hz).Thus, if such a signal is present, it may mask any absence of thecapacitively coupled stimulus signal without additional measures taken.For example, if the noise included an embedded 60 Hz signal and the“expected” or repeated stimulus was 100 kHz, then the algorithm wouldsee correlations at 60 Hz and at 100 kHz. However, since the period ofthe expected stimulus is known (100 kHz), the algorithm may simply beconfigured to discard detected correlations that do not have a spacingthat represents the stimulus frequency of 100 kHz. Of course, if thenoise is a multiple of the stimulus frequency (or repetition rate), thenobviously, a simple solution is to use a different stimulus frequency.An algorithm could be implemented to send a stimulus frequency(s) at afrequency that is not related to any expected noise sources. One suchimplementation is the use of a “chirp” signal (i.e., a signal thatincreases in frequency over time) repeated many times that would notfavor any one frequency. The correlation algorithm would seecorrelations at the rate (or spacing) of the repetition rate of thechirp signal.

The use of auto-correlation may be advantageous in systems where a lotof variation in the transfer functions through the probe plate of eachof its capacitive coupling probes exists. A cross-correlation algorithmrequires the “expected” signal used the in cross-correlation algorithmto be a “typical” transfer function through the sensor plate. Sinceauto-correlation does not need to generate an “expected” signal tocorrelate (since the signal is simply correlated with itself, thedifferences in the transfer function variation between the variousplates, integrated circuits, and different assemblies is irrelevant.

In the preferred embodiment, the stimulus 27 is a coded pulse-trainsequence. If the stimulus 27 is a coded pulse-train sequence, thencorrelation of the digital representation 25 of the capturedcapacitively coupled signal 23 with the expected digital signature ofthe stimulus is performed using cross-correlation. The signal processingis similar to that for processing the square-wave except that the searchfor a peak correlation response must occur over the length of the codesequence. The search must find not only the relative phase between thesource and detected signals; it must also find the relative timing inthe code sequence. Since the amount of shift from the stimulus 27 to thecapacitively sensed signal 23 is well bounded (stimulus-to-detectiondelay≦to one bit length), this timing is relatively easy to recover.

The use of a coded pulse-train sequence as the stimulus 27 offersseveral advantages. First, the coded pulse-train sequence techniquerejects spurious signals at the clock frequency of the pulse train. Onlya signal that correlates with both the clock frequency and the codesequence will generate a response.

Another advantage is that the code sequence technique allows multiplestimulus signals to be used at the same time—even when detected with thesame sensor. By using code sequences that are orthogonal to each other(have a correlation of 0), the post acquisition processing can look formultiple independent responses corresponding to the different codes. Theability for parallel testing provides an obvious advantage in speed overthe square-wave method.

When using a coded pulse-train sequence as the stimulus 27, the systempreferably utilizes a well-known modulation technique known as directsequence modulation. This technique is described in detail in Dixon, R.C., “Spread Spectrum Systems”, Wiley-Interscience, 1976, pp. 1–27;supra. In a preferred embodiment, the tester 300 would use directsequence modulation as described therein.

FIG. 19 is a flowchart illustrating an example general correlationalgorithm 160 that may be implemented by the signal correlator 60 in theembodiments of FIGS. 2, 10, 12, or 15, or correlation function 370 ofFIG. 17. In a step 161, the algorithm 160 receives a digitalrepresentation 25 of a capacitively coupled signal 23 recovered by aprobe 20, 320 a–320 c. The algorithm 160 sequences through a set ofoffsets, attempting to correlate the retrieved digital representation 25of a capacitively coupled signal 23 with an expected digital signature.To this end, the algorithm 160 determines in step 162 whether existingshift offsets remain to be processed. If so, an unprocessed shift offsetis selected in step 163, and the retrieved digital representation 25 ofthe capacitively coupled signal 23 is shifted by the selected shiftoffset in step 164. The algorithm 160 attempts to correlate the shifteddigital representation of the capacitively coupled signal with theexpected digital signature in step 165. If correlation no is found, asdetermined in step 166, steps 162 through 166 are repeated until eithercorrelation is found, or all shift offsets have been processed. Ifcorrelation is found in step 166, the algorithm 160 may terminate, as ina go/no-go test.

Alternatively, as indicated by the steps 167 through 171, the strengthof the correlation may be determined and used to find the level of themaximum strength correlation level. In this embodiment, a maximumstrength value is initialized to zero in step 170 prior to theprocessing of the digital representation 25 of a capacitively coupledsignal 23 in steps 162 through 166. Then, when a correlation is found,as determined in step 166, the strength of the correlation is calculatedin step 167. The calculated strength value is greater than the presentmaximum strength value, as determined in step 168, the present maximumstrength value is replaced with the calculated strength value in step169. Steps 167 through 169 are repeated at the end of each iteration ofthe loop (steps 162 through 165). When all shift offsets have beenprocessed, the maximum strength value contains the peak correlationmagnitude found. The peak correlation magnitude may be compared to apredetermined minimum correlation threshold in a step 171, from which adetermination may be made in step 172 as to the integrity of theconnection associated with the processed capacitively coupled signal.

Although this preferred embodiment of the present invention has beendisclosed for illustrative purposes, those skilled in the art willappreciate that various modifications, additions and substitutions arepossible, without departing from the scope and spirit of the inventionas disclosed in the accompanying claims. It is also possible that otherbenefits or uses of the currently disclosed invention will becomeapparent over time.

1. An electrical integrity testing apparatus for testing electricalintegrity of nets on a circuit under test, comprising: a stimulatingprobe couplable to a first end of a net of interest on said circuitunder test; a signal generator couplable to said stimulating probeoperable to generate a known oscillating digital reference signal; acapacitive sensing probe operable to capacitively couple a signal from asecond end of said net of interest when said known oscillating digitalreference signal stimulates said first end of said net of interest; anda signal correlator which performs signal correlation on a digitalrepresentation of said capacitively coupled signal with said knownoscillating digital reference signal based only on said capacitivelycoupled signal and an expected digital signature of said knownoscillating digital reference signal, said signal correlation indicatingwhether an open defect exists on the net of interest.
 2. An apparatus inaccordance with claim 1, comprising: a classification functionresponsive to correlation results generated by said signal correlator toclassify said net of interest into one of a plurality of differentlevels of conductive integrity.
 3. An apparatus in accordance with claim1, further comprising: a filter which filters signal components fromsaid capacitively coupled signal outside a predetermined frequencyrange, said predetermined frequency range comprising at least a range offrequencies of said known oscillating digital reference signal.
 4. Anapparatus in accordance with claim 1, wherein said known oscillatingdigital reference signal comprises a coded pulse-train sequence and saidsignal correlation comprises cross-correlation of said digitalrepresentation of said capacitively coupled signal with an expecteddigital signature of said coded pulse-train sequence.
 5. An apparatus inaccordance with claim 1, wherein said known oscillating digitalreference signal comprises a square-wave pulse train and said signalcorrelation comprises auto-correlation of said digital representation ofsaid capacitively coupled signal with itself.
 6. An apparatus inaccordance with claim 1, wherein said known oscillating digitalreference signal comprises a square-wave pulse train and said signalcorrelation comprises cross-correlation of said digital representationof said capacitively coupled signal with en expected digital signatureof said square-wave pulse train.